Communication system



I Jan. 22, 1946. R. A. CLARK, JR

COMMUNICATION SYSTEM Filed June l0, 1944 4 Sheets-Sheet l ATTORNEYS R. J K mm M V yA m m Y E B O P R l @i /lmw qm/dll. l Il 1| Nm l- I M Nif, 3m een# 1 mm /H N gli m p zoo? jd. r2 .llc J. NQ* om mm l. 20.25 2925 2955 Z055 eom. N zoom m .Nlw W F E (L13 zommrm Jan. 22, 1946.

R. A. CLARK, JR l COMMUNI CATION SYSTEM Filed -June 10, 1944 4 Sheets-Sheet 2 FIG. 2

VEHICLE STATION ROBERT A CLARK JR BY 31u-KQ? (OMM, v 27mm ATTORNEYS Jan. 22, 1946. R. A. CLARK, JR

COMMUNICATION SYSTEM Filed June l0, 1944 4 Sheets-Sheet 3 y u fw@ ROBERT A. CLARK JR BY A okomkmo.

Jan. 22, 1946. R, A CLARKy JR COMMUNICATION SYSTEM Filed June 1o; 1944 J4 Sheets-Sheet 4 INVEN TOR.

ATTORNEYS u ROBERT CLARK JR.

BY 1 MM @ma v n mahjong om gs v wm mijn 5. muso@ o o VQ $.22 MOGEN@ Patented Jan. 22, 1946 UNITED STATES PATENT,OFFICE COMJVIUNICATION SYSTEM Robert A. Clark, Jr., Chicago; Ill.

' 'Application June 10, 1944, Serial No. 539,672

Claims.

The present invention relates to communication systems and more particularly to an improved system for communicating between one or more stations of xed location and a vehicle moving along a predetermined route.

Two-way space communication between a moving vehicle, such, for example, as a train, moving along a xed route and one or more stations located along the route is of comparatively recent origin. It is recognized that in such a system the rangeof space communication must necessa-rily be restricted in order to minimize interference with long range radio transmission on assigned frequency channels. In at least one proposed system this has been accomplished by employing inductive coupling between the vehicle station and the way station to provide signal carrier energy transfer between the stations.'

The usual system arrangement of this character, however, makes no provisions for in anyway utilizing the communicating or signaling facilities which may parallel the vehicle route iny establishing communication channels between the vehicle station and the various points along the route. Thus the usual railway line is conventionally paralleled by telephone communication facilities between one or more centrally located dispatcher stations and control towers or way stations spaced along the line. Such railway lines are also usually paralleled by telegraph and power lines, yet these facilities are not normally used in establishing communication channels between the vehicle stations, the way stations, and the dispatcher station or stations.

It is an object of the present invention. therefore, to provide a system for communicating between a vehicle proceeding along a predetermined route and one or more stations of fixed location along the route, which ellectively utilizes to the best possible advantage existing communicating facilities which may parallel'the route.

It is another object of'the invention to :provide an improved system of thecharacter described which may be employed to furnish two-way communication between a vehicle station and a dispatcher or master station over channels which respectively include different way stations located along the route over'which the vehicle is travelling.

According toa further object of theinvention, the vehicle and way stations are each equipped with space communication equipment of limited communication range, and the way stations are so spaced along the route that a substantial overlap is provided ,between the communicationV ranges of adjacent way stations, whereby the vehicle station is successively moved into the ranges of communication of successively adjacent way stations as the vehicle proceeds along the predetermined travel route.

According to yet another ,object of the invention, the space communication equipment at the various way stations is commonly coupled to a single signal radiating and `pick-up line paralleling the vehicle route.

In accordance with a still further object of the invention a common, general purpose communication channel is utilized to interconnect the dispatcher or master station with the different way stations, and each way station includes facilities selectively controllable over the common channel for operatively associating the space communication equipment thereat with the common channel for the radiation of master sta.

tion originated signals at the vehicle station and the relaying of vehicle station originated signals to the master station.

According to still another object of the invention, each way station is also provided with facilities controllable from the master station over the common channel for selectively reversing the direction in which signals may ibe transmitted between the master station and the vehicle station when the spa/ce communication equipment thereat is being utilized to communicate between the master and vehicle stations..

In accordance with yet another object of the invention, each way station is also equipped with means controllable from the vehicle station over a space link therebetween for establishing a monitoring channel between the vehicle station and the common channel.

More generally, it is an object of the present invention to provide an improved system of the character described which includes relatively simple and easily operable facilities for providing communication between a vehicle station and any one of a number of way stations or a dispatcher station.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which: y

n Fig. 1 diagrammatically illustrates an improved communication system characterized by the features of the present invention;

Fig. 2 schematically illustrates the vehicle station equipment which may be utilized in the system shown in Fig. 1; and

Figs. 3 and 4 when laid end to end in the order named illustrate the details of the equipment provided at one of the way stations included in the system shown in Fig. l.

Referring now to the drawings and more particularly to Fig. 1 thereof, the communication system there schematically illustrated is utilized to provide for voice signal transmission between a dispatcher station I5 and any one of a plurality of tower or way stations I8, I1, I8, I9, etc., located at spaced points along a railway track system III. It is also used to provide for voice signal transmission between any one of the named stations and the vehicle stations IIa, IIb, I2a and I2b of two trains II and I2 which may be moving in the same or different directions along the track system I0. As indicated by the dash line rectangles, each train moving along the track I may be equipped with at least two vehicle stations. These stations may be utilized for intratrain space communication or for communication between the stations of two different trains moving in the same or opposite directions along the track system, and the two stations of each train are usually provided in the engineers cab and the caboo'se or last car of the train. In accordance with conventional practice, the track system III is paralleled by a line I3, which is insulated from ground and may, for example, be used for power transmission or telegraphic signaling purposes. This line is commonly associated with the various way stations to function as a signal radiating and pick-up circuit by means of which voice signals modulated upon an appropriate carrier may be transmitted in either direction between the various way stations and the vehicle stations of trains moving along the track system I0. In order to provide for telephone communication between the dispatcher station I and the various way stations I8, I'I, I8, I8, etc., these way stations are also commonly associated with the dispatcher station by means of a two-conductor telephone line I4. Each way station is provided with a telephone subset which is bridged across the line I4.

Briefly considered, the equipment provided at the dispatcher station I5 comprises a telephone substation circuit 32, facilities for selectively seizing and operatively associating the different way stations with the common signal channel I4, and a press-to talk key 34 for controlling the equipment provided at a selected way station to condition the same for the radiation of voice signals originating at the dispatcher station. The substation circuit 32 may be of any conventional form although preferably is of the well-known antiside tone type conventionally utilized at telephone substations. Ir.' lieu of a handset, it preferably employs a loud speaker and microphone as the reproducing and transmitting elements. The selective calling facilities include a plurality of oscillators 20', 2|, 22 and 23, individually corresponding to the way stations I8, I'I, I8 and I9, and the output voltages of which may be selectively impressed across the conductors of the line I4. Relays 28, 29, 30 and 3|, arranged for selective energization by means of associated keys 24, 25, 26 and 21, are provided for selectively connecting the output terminals of the four identiiied oscillators across the conductors of the line I4. Current for energizing the enumerated relays may be derived from the usual storage battery 35 of appropriate voltage.

In order that the call signal receiving channels respectively provided at the way stations I6, I1, etc., and of the specific character described below, may distinguish between voice signals and call signals transmitted over the line I4, the oscillators 20, 2l, 22 and 28 are adjusted to have output frequencies above the frequency range normally utilized for voice signal transmission over the line I4. Thus the voice signal transmission frequency range may be limited not to exceed an upper frequency limit of 3,000 cycles, in which case the oscillators may have output frequencies of 3,500 cycles, 3,750 cycles, 4,000 cycles and 4,250 cycles, respectively. This frequency spacing between the output frequencies of the oscillators is entirely adequate for reliable selective response of the call relays respectively provided at the way stations.

Referring now more particularly to the various way stations, it is pointed out that the equipment provided at these stations is of identical arrangement. Accordingly, only that located at the way station I8 has been illustrated in detail. In brief, the equipment at this station comprises a. space signal transmitter 48, a space signal receiver 49, a call signal receiving channel 51, a call signal rectifier 62, a loud speaker 65, a microphone or transmitter 64 of the sound powered type, and a relay network controllable in the manner described below to perform the switching operations which are required in utilizing the described equipment to communicate between the dispatcher station or a vehicle station, or to relay signals between a vehicle station and the dispatcher station. More specifically, the transmitter 48 comprises an audio amplifier stage 50 and a, carrier producing oscillator 52, both working into a modulator stage 5I, the output of which may be impressed between the line I3 and ground through the contacts of a switching relay RI I0 and an appropriate coupling network 56. The receiver 49 is comprised of a two-stage signal modulated carrier ampliier 53 having its input circuit normally coupled to the line I3 and ground through the contacts of the switching relay RI I8 and the coupling network 58, and its output circuit coupled to the input side of a detector and automatic volume control stage 54, the output terminals of which are coupled to the input circuit of a power amplifier 55. Since the transmitter 48 and receiver 49 are substantially conventional in circuit arrangement, the individual circuit elements thereof have not been specifically identified in the drawings. Also, the coupling network 56, through which signal voltages may be transmitted in either direction between the line I3, the transmitter 48 and the receiver 49, may be of various conventional arrangements depending upon the particular character of the line I3. lRegardless of its character, however, this coupling network is arranged to exclude from the transmitter 48 and the receiver 49, the voltages appearing on the line I3 in the normal use thereof, and is also arranged to provide high impedance paths between this line and ground at the frequencies of the currents traversing the line in the normal use thereof.

For the purpose of utilizing the described equipment at the way station I8 to permit a vehicle station operator to monitor the common line I4 and to then audibly call the attendant at the dispatcher station I5 if the line is idle, three relays RI30, RI40 and RI50 are provided at this way station. These relays are arranged to be energized by transmitting a call signal through the receiver 49 to a call signal rectifier network 62, the equipment provided in the latter'networkbeing identical with that included in the channel 51. The input side ofthis network is coupled to the output side of the power ampliler 55 through a voice frequency blocking filter 56 of conventional design. This fllter is arranged to pass the call signal, which may be radiated at afrequency of 3.250 cycles, for example, outside of the band utilized for voice transmission. Facilities including a blocking filter 51 may be provided at the out- I put side of the amplifier 55 for preventing this call* signal fromv being impressed upon the loud speaker 65 or the line I4. The channel 51, utilized in seizingthe way station I8 from the dispatcherV station I5, comprises a one or two section band pass filter 58, a call signal amplifier 59, a detector 80 and a control tube 6I connected in tandem in the order named'between the line I4 and a call relay RI00. 'I'he band pass filter 58 is preferably sharply tuned to pass only a received signal of. 4,000 cycles, thereby excluding from the suc- .ceeding stages of the channel 51 the call signals corresponding to the other way stations as well as voice signals which may be transmitted over the line I4. Inthis regard, it is noted that the corresponding filters as provided at the other way stations-'are respectively tuned` to pass only frequencies of 3,500 cycles, 3,750 cycles and 4,250 cycles, whereby the stations may only be seized when the respective corresponding keys 24, and 21 are operated at the dispatcher station. In addition to the equipment thus far described, the

vway station I8 is also equipped with a dispatcher station call key I 24 and a switching relay RI20, arranged for selective energization through the contacts of a-press-to talk key 66, which is utilized selectively to associate the'transmitter 48 and the receiver 49 with the line I3. Voice signals are repeated from the line I4 tothe transmitter 48 and from the receiver 49 to the line I4 through a coupling transformer 63 having a winding 63h bridged across'the conductors of this line. The way station is further provided with a direct current source, not shown, of appropriate voltage having a positive terminal connected to ground and a negative terminal connected to all of the relay terminals identified by the negative polarity sign. This source is also utilized to supply current to the cathode heaters of the tubes embodied in the units 51, 62, 48 and 49. Anode voltages are supplied to these tubes from a 125 volt direct current source, the negative terminal of which is grounded and the positive terminal of which is connected to the conductor |26.

The vehicle stations Ila, IIb, I2a, I 2b, etc., are equipped with space communication equipment of identical arrangement. In brief, the equipment provided at the vehicle station IIa comprises a microphone 4I and a loud speaker 40 which may be selectively associated with a signal pickup and radiating circuit 36 through a signal modulated carrier receiver 38 and a signal modulated carrier transmitter 39, respectively. More specifically, the input terminals of the receiver 38 are normally associated with the signal radiating and pickupv circuit 36 through a coupling network 31 and the contacts of the switching relay 45. The output terminals of the transmitter 39 are also adapted to be coupled to the signal pickup and radiating circuit 36 through the contacts ofthe switching relay 45 and the coupling network 31. The receiver 38 and transmitter 39 may correspondfprecisely in circuit arrangement to the receiver 49 and transmltter 48, respectively, of the equipment provided at way station I8. The vehicle station equipment further comprises a press-to talk key 44 for selectively controlling the .energization of the switching relay 45, together with facilities for modulating the carrier outputof Vthe transmitter 39 with a way station call signal. 'I'hese facilities comprise a signal oscillator 42 having an output frequency which corresponds tothe tuning .of the filter 56 and the filter section provided in the call signal rectifier 62 of the illustrated way station I 8, i. e., of the order of 3250 cycles. More specifically, the output terminals of the oscillator 42 are coupled to the input side of thetransmitter 39 Vthrough a coupling condenser 48, and this oscillator is arranged for selective operation under the control of a call key 43 having contacts for also controlling the switching relay 45. Suitable current supply facilities, including a battery or other current source having its positive and negative terminals connected to ground and to the negative terminal ofthe relay 45, respectively, are providedfor supplying current to the various elements of the described vehicle station equipment.

As indicated above, the radius of space transmission from any one of the vehicle and way stations must necessarily be restricted in order not to yinterfere with radio communication upon assigned frequency channels. To this end, straight inductive coupling of the general character described by L. O. Grondahl and P. N. Bossart in Railway Signaling for December l1943, starting at page 665, is preferably utilized -between the signal pickup and radiating circuits 36 individual to the vehicle stations and the lineground radiating and pickup circuit `common to the several way stations. Also, the pickup and radiating circuits 36 individual to the various vehicle stations are preferably of the general character described in this article. With this arrangement, the range of transmission from -eac'n vehicle stationl is of course definitelyrevstricted to a short distance, which may vary -of a train traveling along the track system I 0 may be very short, not exceeding a few hundred yards, such that the space link of a signal chan- -nel between the vehicle station and one of the way stations is relatively short. The conducive portion of the channel may, on the other hand, be relatively long dependingupon the distance sepa-rating the vehicle station from the way station. It is the attenuation in the conductive portions of such signal channels which must be considered in determining the spacing between the way stations. In accordance with one feature of the present invention the way stations are so spaced along the route followed by the track system I0 that 100 percent overlap is provided between the lsignal pickup and radiating ranges of adjacent way stations. transmitters 39 and 48 of the vehicle and wey stations setto produce' signals of given field strength, the signal pickup and radiating range of the way station I8, for example, in respect-to a vehicle station which is being moved along the Thus, With the track system IU, completely overlaps the corresponding ranges of the two adjacent way stations I1 and I9. Entirely satisfactory communications between the way stations and vehicles travelling along the track system I may be obtained by utilizing a spacing of from 25 to 50 miles between the adjacent way stations.

From the above description of the system, it will be understood that the space communication equipment provided at the vehicle station IIa may be utilized by the attendant at this station to communicate with the attendant at the station IIb on the same train or with either of the stations I2Ia and I2b of another train travelling along the track system in proximity to the train II. Thus, if the attendant at the station IIa desires to converse with the attendant at the station IIb, he may operate the press-to talk key 44 to complete an obvious circuit for energizing the switching relay 45. This relay, in operating, opens its contacts 46 to disconnect the input terminals of the receiver 38 from the coupling network 31 and closes its contacts 41 to connect the output terminals of the transmitter 39 to the pickup and radiating circuit 36 through the coupling network 31. After the relay 45 has thus operated, the attendant at the station IIa may speak into the microphone 4I thereby to effect radiation of a voice modulated carrier by the radiating and pickup circuit 36. This signal is picked up by the circuit 36 rprovided at the vehicle station IIb, and is transmitted through the coupling network 31 and receiver 38 of this station to the associated loudspeaker 40 for reproduction. After the attendant at station IIa has completed his statement to the attendant at the station I Ib, he may release the key 44 in order to condition the equipment at the station IIa for the reception and reproduction of the reply transmitted from the called station IIb. Incident to the release of the key 44, the relay 45 is deenergized. In releasing, this relay opens its contacts 41 and closes its contacts 46, thereby to disconnect the transmitter 39 from the coupling network 31 and to establish a channel for transmitting the reply signal to the input side of the receiver 38. It will be thus understood that through selective actuation of the press-to talk keys 44 respectively provided at the two stations I Ia and I Ib, a two-way conversation may be held between the attendants at these two stations. Space communication may be carried on between the vehicle station IIa and the vehicle station I2a, for example, of the train I2 in exactly the same manner.

Intratrain communication or intertrain com munication effected through the use of the vehicle station equipment in the manner just explained, may be monitored at the way stations within the radius of transmission of the vehicle stations involved in such communication. Thus, if the train II is travelling along the track I0 within-the radius of reception of the way station I8 at the time the facilities of the two stations IIa and IIb are being utilized in communicating between these vehicle stations, the signals radiated from these stations are picked up by the common pickup and radiating circuit I3 and are transmitted through the coupling network 56 and the contacts II I of the switching relay RI I0 to the input side of the receiver 49. In this receiver, the voice modulated carrier is amplied through the twostage ampliiier 53 and detected in the detector and A. V. C. stage 54, following which the voice frequency components of the signal are amplified in the power ampliner stage 55 and delivered through the filter network 51 and the contacts I 22 of the switching relay R I 26 to the loud speaker for reproduction. At this point it is noted that the A. V. C. circuit embodied in the receiver 49 is of substantially conventional arrangement, and functions to vary the gain through the ampliiier 53 inversely in accordance with the strength of the received signal carrier, whereby the volume of the reproduced signal is maintained substantially constant regardless of the strength oi the carrier appearing at the input side of the receiver.

It will thus be apparent that as the train II, for example, moves along the track system I0, it is successively moved into the ranges of reception of the various way stations I8, I1, I6, etc., so that communication between the attendants at the stations IIa `and IIb may be successively monitored by the attendants at the diilerent way stations. It will also be understood that the attendant at the vehicle station Ila, for example, may converse directly with the attendant at any way station within the radius of transmission of the vehicle station by the simple expedient of calling this way station directly. Thus, if the attendant at the way station II desires to converse with the attendant at the way station I8 while the train II is travelling within the range of communication between these two stations, he may simply throw the press-to talk key 48 to the talk position and call the way station I8, either by name or number. To reply, itis necessary for the attendant at the called way station I8 to actuate the non-locking press-to talk kry 66, thereby to complete an obvious circuit for energizing the switching relay RI20. In operating, this relay closes its contacts |23 to complete an obvious circuit for energizing the associated switching relay RI I0. At its contacts |22, the relay RI2II opens the connection between the loud speaker 65 and the output circuit of the receiver 49. At its contacts I2 I, the relay RI20 completes anv obvious path for bridging the microphone 64 across the input circuit of the transmitter 48.

The relay RI III, upon operating, opens its contacts I I6 to interrupt the path over which anode potential is normally supplied to the four tubes of the receiver 49, thereby to render this receiver inactive. At its contacts I I5, the relay RI III oompletes an obvious path for impressing positive anode potential upon the anodes of the three tubes included in the transmitter 48, thus rendering this transmitter active. At its contacts II4, the relay RI I0 opens the path over which voice signals may be transmitted from the line I4 to the loud speaker 65 and the input circuit of the transmitter 48. At its contacts II3, the relay RI IIJ prepares a path for transmitting voice signals from the microphone 64 to the line I4. At its contacts III, the relay RIII) opens the connection between the coupling network 56 and the input terminals of the receiver 49. At its contacts II2, the relay RI Ill connects the coupling network to the output terminals of the transmittei- 48. Thus, the equipment provided at the way station I8 is conditioned for the radiation of voice modulated carrier signals to the vehicle station I Ia. More specifically, when the contacts II5 of the relay RI I0 are closed to apply an operating potential to the anode of the oscillator 52, operation of this oscillator to generate a high-frequency signal carrier is initiated. Thereafter and when the attendant at the station I8 speaks into the loud speaker 65, the genand the coupling network 56 for radiationto the pickup and radiating circuit 36 of the vehicle station Ila. At this point, it is specically noted y that the voice signals generatedthrough operation of the microphone 64 in the manner Just explained are vnot impressed upon the line I4. Thus, with only the two relays Rllll andRI20 operated, the circuit over which'voice signals may be transmitted from thel microphone 64 to the line I4 is held open at the contacts IUI, |52 and |42 of the three de-energized relays -RI I0, R|50 and RI4II. It is also noted that the voice signals which are radiated from the vehicle station IIa and transmitted through the receiver 49 to the loud speaker 65 for reproduction d ng a period when the two relays RI Ill and RI20 are de-energized, are not repeated or relayed to the line I4. Under such circumstances, the connection between the output terminals of the receiver 43 and the line I4 is also held open at contacts Illl, I 52 and |42 of the three de-energized relays Rlllll, RI50 and RIM).

After the attendant at the way station I8 has completed his reply to the attendant-:at the vehicle station Ila, he may release the self-restoring press-to talk keyv 64, thereby to condition the associated equipment for the reproduction of the voice signals which may be radiated from the station IIa in response to his reply. Incident to the release of the key 66, the two relays RI20 and RIIIl are successively de-energized in 1^ order named. When these relaysrestore, transmitter is rendered inactive and the receiver active, the coupling network 56 is switched frffem a connection with the transmitter 48 to a connection with the receiver 49, the connection between the microphone 64 and the transmitter 48 is broken, and the connection between the loud speaker 65 and the receiver 43 is restored, all in a manner which will be fully apparent from the above explanation. It will thus be clear that through selective actuation of the press-to talk keys 44 and 66 respectively provided at the stations IIa and I8, a two-way communication may be held between these two stations so long as the train II is travelling within the range of communication extending to the way station I8.

Assuming now that the attendant at the vehicle station |Ia desires to converse with the dispatcher at the station I5, the call to the dispatcher station may be initiated by actuating the call key 43 for a short interval and then releasing the same. When this key is actuated, the relay 45 is energized through the contacts 43h and the start circuit for the signal oscillator 42 is completed at the contacts 43a. With the relay 45 energized and the signal oscillator 42 operating, acarrier modulated at the call frequency of the oscillator 42 is radiated from the station IIa. Assuming that the call is thus initiated while the train is travelling within the range of communication between the stations IIa and I8, the modulated carrier is transmitted through the receiver 49 and theffllter network 56 to the input side of the call signal rectifier 62. In this regard it is pointed out that the filter 51 has the function of blocking the received call signal against transmission to the loud speaker 65. Similarly, the ilter 56 has the function range from being transmitted tothe 'inputfside of the call signal rectifier 62. k' .It isy pointed out further that a common call signal frequency, preferablyV of the yorder of 3,250 cycles, .is assigned forcalling any way station from any vehicle station. l In other words, the signal oscillators 42 as provided at all vehicle stations are set to operate at this` frequency, andthe lters 56 as provided at the respective way vstations are designed to pass only lthis frequency.

. The call signalv as transmittedy through the filter 56 to the rectifier 62 is rectied through thisv filter and is utilized to eiect a rise in the current traversing the series connected windings of the two relaysRI30 and R|40of suiiicient magnitude to cause these relays to operate. Atits contacts,-|4|, the relay R|40 opens a point in the circuit for energizing the switching relay RI I0. At its contacts |42, the relay`R|40 bridges the loud speaker 65 across the winding 63a of the transformer 63 and completes a path for connecting the indicated transformer winding across the output terminals of the receiver 48. The relay R|3|l, upon operating, closes its contacts |3I to shunt its own winding with the series connected condenser |34 and resistor |35, thereby to render itself slow to release. At its contacts |32, the relay R|3ll completes an obvious circuit for energizing the, slow to release relay RISII. The latter relay, upon operating, closes its rcontacts |52 to shunt the contacts |42 in the circuits for coupling the line I4 to the input side of the transmitter 46 or the output tery minals of the receiver 49. At itsv contacts I 5|,

the' switching relay RI I0.

With the three relays R|30, R|40 and R|50 thus operated, and with the relay 45 operated at the vehicle station IIa, a channel including the transmitter 39, the contacts 41, the coupling network 31, the space link between the stations IIa and I6, the coupling network 56, the receiver 49, the contacts |I4, |42 and |52, and the coupling transformer 63, is provided for transmitting voice signals from the vehicle station IIa to the line I4 and over this line to the dispatcher station for reproduction. If, therefore. the call is suillciently urgent, the attendant at the station I|a,may call the dispatcher by speaking into the microphone 4I and identifying the train II, without waiting to monitor the line in order to determine the idle or busy condition thereof. On the other hand, if the call is not urgent, the normal procedure is for the attendant at the station to monitor the line I4 for a short monitoring period before actually calling the dispatcher. To this end, the key 43b is released, thereby to terminate the transmission of the call signal from the station IIa to the way station I8. Whenthls signal is terminated, the current traversing the winding of the relay RI40 is rapidly decreased to effect the release of this relay. In restoring, the relay R|4Il opens its contacts |42 to interrupt the shunt across the contacts |52. At its contacts I4I, the relay RHI) completes the prepared circuit through the colnltacts I 5| for energizing the switching relay R 0.

The relay RI I0, upon operating, opens its contacts I I I to interrupt the channel over which the signal modulated carrier was delivered to the input side of the receiver 49. Due, however, to the described slow to release characteristics of the two relays R|40 and RI50, these relays stay operated for several seconds after the delivery of the call signal voltage to the rectifier 82 is interrupted. At its contacts II2, the relay RI IO connects the coupling network S8 to the output terminals of the transmitter 48, thereby to prepare the equipment for the radiation of voice signals traversing the line I 4. At its contacts i I4, the relay RI O interrupts the path bridging the transformer winding 83a across the output side of the receiver 49. At itsccntacts II3, the relay RI IO completes a path through the parallel connected contacts I52 and |42 for bridging the winding 63a across the input terminals of the amplifier 50. At its contacts IIS, the relay RI IO.

-opens the circuit over which anode current is delivered to the tubes of the receiver 49, thus rendering this receiver inactive. At its contacts I I5, the relay RI IO completes the previously described circuit for delivering anode current to the tubes of the transmitter 48, thus rendering this transmitter operative. From the above explanation, it will be understood that upon operation of the switching relay RI IO, the receiver 49 is rendered inactive, and the transmitter 48 is rendered operative to radiate voice signals appearing upon the line I4 as a result of the use of this line in communicating between dispatcher station I and other way or vehicle stations disposed along the trackIO. Thus if the line I4 is busy, the voice signals appearing thereon are impressed upon the input terminals of the transmitter 48 through the coupling transformer 63, the contacts |52, and the contacts II3. Such voice signals are modulated upon the signal carrier generated through operation of the oscillator S2 and are delivered to the line I3 for radiation to the vehicle station IIa through the contacts II2 and the coupling network 58. It will be apparent, therefore, that if the line I4 is busy at the time the call to the dispatcher station I5 is initiated at the vehicle station II, the attendant at this vehicle station may ascertain this fact during the short monitoring period which is determined by the release time of the relays RI3O and RI40. After a period of from ten to twenty seconds,v starting with the operation of the relay RI IO, the relay RI3O is sufficiently de-energized to release. Upon restoring, this relay opens its contacts |32 to interrupt the path over which the delay impedances |34 and |85 are connected in shunt with its own winding, thereby to render itself fast to operate. At its contacts |82, the relay RI3O de-energizes the relay RISO. The relay RI5O now restores andcpens its contacts I5I to de-energize the switching relay RI IO.

At its contacts III and II2, the relay RIIO disconnects the line I8 from the output terminals of the 'transmitter 48 and reconnects this line to the input terminals of the receiver 49. At its contacts II5 and IIS, the relay RIIO opens the circuit for delivering anode current to the tubes of the transmitter 48 and recloses the circuit for delivering anode current to the tubes of the receiver 49. Thus, the transmitter 48 is rendered inactive and the receiver 49 is rendered active. At its contacts II3, the relay RI IO opens the described path for bridging the transformer winding 63a across the input terminals of the ampli- Iier 50. At its contacts II4, the relay RI IO completes a path through the closed contacts I52 for bridging the transformer winding 83a across the output terminals of the receiver 49. Thus the equipment provided at the way station I8 is restored to normal.

If upon monitoring the line I4 in the manner Just described the attendant at the vehicle station IIa finds that the line is idle, he may then call theidispatcher at the station I5 by again pressing the key 43 and talking into the microphone 4I. When the key 43 is operated, the relays 45, RISO, RI4O and RISO function in the manner explained above to condition the equipment at the way station I8 for the relaying of voice signals from the station IIa to the line I4. Accordingly, the attendant may speak into the microphone 4I and identify the vehicle station IIa as a calling station at the dispatcher station IS. Upon release of the key 43, the relays RI30, RMO and RISO function to first establish the described monitoring channel and to then restore the equipment at the Way station I8 to normal.

From the above explanation it will be apparent that the described monitoring period is measured -by the release time of the relays RISO and RISO. If desired, the two relays RISO and RISO may be of the dash-pot type, in order to provide for monitoring periods of longer duration than can be obtained by conventional slow-to-release relays. It will also be understood that since the three relays RI30, RMO-and RI 50, as respectively provided at the diiIerent way stations, are arranged to respond to call signals of the same frequency, i. e., 3250 cycles. and the vehicle stations are equipped to transmit call signals of only this frequency, the equipment 'at two of the way stations may be concurrently operated in the exact manner described above when a call signal is radiated from a vehicle station. Thus if the station IIa is located at a point between the two way stations I1 and I8 at the time the above- 4 described call to the dispatcher station I5 is initiated thereat, the equipment at each of the two stations I8 and I'I will be concurrently operated to ,establish two parallel monitoring and call channels between the line I4 and the vehicle station IIa.

As indicated above, if the line I4 common to the several way stations is not busy at the time the above described call is initiated at the vehicle station IIa, the attendant at the latter station may audibly call the dispatcher attending the station IS by speaking into the microphone 4I and identifying his particular vehicle station. The voice currents are modulated upon the carrier output of the transmitter 39 and radiated to the way station I8 where they are picked up by the receiver 49, demodulated in the detector 54, am

plified in the amplifier 55 and transmitted through the coupling transformer 63 and over the line I4 for reproduction by the loud speaker at the dispatcher station I5. They are also delivered from the receiver output circuit through the contacts |22 to the loud speaker 65 for reproduction. Thus the attendant at the way station I8 is informed that the dispatcher is being called from the vehicle station I Ia. Knowing the identity of the vehiclestation IIa, the dispatcher attending the station I5 may, by consulting train schedules, ascertain the approximate location of the train II along the track system IO and thus be infomed as to the proper Way station to seize in order to establish a two-way communication channel between the dispatcher station and the calling vehicle station IIa. In the case under consideration, the dispatcher can readily ascertain that the way station I8 is within the range of communication with the vehicle station IIa. To seize this way station, the dispatcher may actuate the locking call ke'y 26, thereby to ener- Vgize the relay 30. Thisrelay, lin operating, bridges the output terminals of the`4000 cycle oscillator 22 across the conductors ofthe line I4'. 'I'he high'- frequency voltagel is passed through the filter 58 at the way station I8 and is impressed upon the input circuitof the amplier 59. At each ofthe other way stations, the band pass lter 58 functions to prevent the described call signal voltage from appearing in the input circuit of the associated amplifiers 59. The signal voltage appearing at the output side of the illustrated'lter 58 is amplified through the amplifier 58, rectified in the detector 60, and the direct current component thereof is utilized to decrease the bias between the input electrodes of the control tube 6 I, thereby to increase .the current traversing the winding of the call relay RI00. This current increase is suilicient to cause the relay R| to operate. At its contats I 0 the relay RI 00 bridges the transformerwinding 63a across the output terminals of the receiver 49 and prepares-a path for bridging this winding across the input terminals of the transmitter 48. At its contacts |02, the relay RI 00 prepares a circuit for energizing the switching relay RI I0. The station call key 26 is held in its off-normal position to maintain the output terminals of the oscillator 22 bridged across the conductors of the line I4 for the duration of the call to fthe vehicle station IIa. Substantially concurrently with the voperation of this key, the press-to talk key 34 is operated to complete an obvious path for connecting the negative terminalI of the battery 35 to the line conductors I4a and I 4b through opposite halvesof the transformer winding 33a. The'voltage thus applied to the conductors of the line I4 serves to energize the switching relay RI I0 in a circuit which includes the conductors' I4a and I4b in parallel, the two halves of the transformer winding 63h and the contacts |02. In operating, the relay RI I0 prepares the equipment at the way station -|8 for the radiation of Voice signals impressed upon the line I4 in a manner which will be fully apparent from the preceding explanation. Following the operation of the two keys 26 and 34 to eiect the described switching operations at the way station I8, the dispatcher attending the station I5 may speak into the microphone or transmitter embodied in the sub-station equipment 32 to transmit voice signals over the line I4 to the way station |8. With the transmitter 48 active, these signals are modulated upon the carrier output of the oscillator 52 and radiated from the line I3 to the pick up and radiating circuit 36 at the vehicle station Ilia. From this circuit, they are transmitted through the coupling network 31 and the receiver 38 to the loud speaker 40 for'reproduction. It will be noted that since the relay RI I0 is operated, the contacts I|4 are open to prevent the voice signals originating at the dispatcher station I5 from being reproduced by the loud speaker 65. After the dispatcher has answered the call in the manner just described, the key 34 may be restored to normal to de-energize the switching relay RI I0 and thus prepare the equipment at the way station |8 for the transmission of voice signals in the reverse direction, i. e., from `the vehicle station I Ia to the dispatcher station 32. To initiate such voice transmission, it isobviously necessary for the attendant at the vehicle station IIa to actuate the press-to talk key 44 for the purpose of associating the transmitter 39 with the pickup andA radi-V ating circuit 36. More generally, it will be apparent from the above explanation that by ap tion I5. The voice signals originating at the vehicle station are reproduced by the loud speaker 65 and serve to keep the attendant at the way station I8 advised as to the duration'of thecall. After the conversation has been concluded, the dispatcher at the station I5 may release the equipment at the way station I8 byrestoring the call key 26 to normal. In response to this operation, the relay 30 restores to disconnect the output terminals of the oscillator 22 from the line conductors I4a and I4b, thereby to de-energize the call relay RI00.v In releasing, the relay R|00 opens its contacts `|0I tbk interrupt the pathsover which the transformer winding 63a may be selectively bridged across the input terminals of the transmitter 48 or the output terminalsof the receiver 49. Thus, the equipment at the way station I8 is restored to normal. y

The manner in which the communication equipment respectively provided at theA other way stations may be seized on a selective basis through selective operation 'of the call keys 2,4, 25 and 21 will be evident fromv the preceding eX- planation. It will also be understood that if the dispatcher attending the station I5 desires to talk with the attendant at the way station I8, for example, the call key 26 isactuated; to impress the output voltage of the oscillator 22 across the conductors of the line I4 and thus effect operation of the call relay RI 00. With this relay operated and the two relays RI I0 and RI20 released, voice signals transmittedover the line I4 from the dispatcher station I5 are impressed upon the loud speaker 65 through vthe coupling transformer 63 and the contacts` |0I, |I4 and |22. Such signals are not impressed upon the input side of the transmitter 48 for radiation from the way station I4. Thus with only the relay R|00 operated at the station I8, the path for transmitting voice signals from .the line I4 to the transmitter 48 is held open at the contacts II3. To answer the call, it is necessary for the attendant at the Way station I8 to actuate the press-to talk key 66 for the purpose of energizing the relay RI20. This relay, in operating, closes its contacts |23 to energize the switching relay RI I0. At its contacts |2I and |22 the relay RI20 disconnects the-loud speaker 65 from the transformer Winding 63 and prepares a path for bridging the microphone 64 across this transformer winding. The relay RI I0, in operating, closes its contacts I|3 to'connect the microphone 64 across the transformer winding 63a. The remaining circuit switching operations of the relay R||0 are without effect. Following operation of the relays RI20 and RI I0, voice signals maykobviously be transmitted from the microphone 64 through the transformer 63 and overthe line I4 for reproduction by the loud speaker provided at the dispatcher station I5. It will be understood, therefore, that by selective actuation of the press-to talk key 66 at the called way station I 8, voice'signals may be transmitted in either direction between the two stations I5 and I8. In this regard it will be noted that with the two relays RI I0 and RI20 operated, the transmitter 48 is conditioned for operation and voice signals developed through operation of the microphone 64 are impressed upon the input circuit of this transmitter. Thus, the voice signals are radiated by the transmitter 48 and are received at the vehicle station of any vehicle moving along the track system I Within the range o f transmission of the Way station I8. The attendant at any such vehicle station is thus informed that the way station I8 is busy.. Here again, the release of the equipment at the way station I8 is under the control of the dispatcher attending the station I5, such release being effected by restoring the call key 26 to normal to effect the deenergization and release of the call relay RIUII. In a manner entirely similar to that just described, the dispatcher attending the station I may selectively call the attendants at any one of the way stations I6, I1, I9, etc., associated with the line I4.

It will be noted that when the equipment at the way station I8 is not in use on calls involving the dispatcher station l5, all of the relays provided thereat are deenergized. With the relays.

RIUII, RI5I| and RI40 restored, the contacts I|||, |52 and |42 are open to prevent voice signals from being transmitted from the line I4 to the loud speaker 65 or the input circuit of the transmitter 48 and to prevent voice signals from .being transmitted from the output circuit of the receiver 49 or the microphone 64 to the line I4. Thus, the line I4 is guarded against receiving voice signals which might interfere with the use thereof by the dispatcher attending the dispatcher station I5. If the attendant at the way station I8, for example, desires to monitor the line I4 for the purpose of determining the idle or busy condition thereof, he may actuate the key |24 to bridge the loud speaker 65 and the input circuit of the transmitter 48 across the transformer winding 63a over a path which includes the contacts ||4 and the contacts |22. With this monitoring connection established, voice signals appearing upon the line I 4 are obviously transmitted to the loud speaker 65 for reproduction. If upon establishing this monitoring connection, the attendant at the way station I8 ascertains that the line I4 is idle, he may call the dispatcher station I5 by actuating the press-to talk key 66, thereby to effect the sequential operation of the two relays RI and RI I0 in the manner described above. With these two relays operated, the loud speaker 85 is disconnected from the transformer Winding 63 and the microphone 64 is bridged across this winding to complete a channel over which voice signals may be transmitted to the dispatcher station I5 for reproduction. After insti tuting a call in the manner just described, the attendant at the way station I8 may, by selective operation of the key 66, condition the equipment at the way station I8 for the reception of signals received over the line I4 or the transmission of signals to this line, thereby to provide for a two-way communication with the dispatcher station I5. After the call is completed, the key |24 should be restored to normal in order to interrupt the monitoring connection between the loud speaker 65 and the line |4.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein which arey within the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A system for communicating between a dispatcher station and a vehicle moving along a predetermined route, comprising way stations provided at spaced points along saldi-cute and each including space transmitting and receiving means, signal radiating and pickup means common to the transmitting and receiving means at said way stations, a station carried by said vehicle and including space transmitting and receiving means, a signal channel commonly associating said way stations with said dispatcher station, and means for selectively establishing communication channels between said dispatcher station and said vehicle station over routes which respectively include different ones of said way stations.

2. A system for communicating between a dispatcher station and a vehicle moving along a predetermined route, comprising way stations provided at spaced points along said route, a station on said vehicle, means providing for space communication between said vehicle stationand different different ones of said way stations when said vehicle is disposed at different points along said route, and means for selectively establishing communication channels between said dispatcher station and said vehicle station over routes which respectively include different ones of said Way stations.

3. A system for communicating between a dispatcher station and a vehicle moving along a predetermined route, comprising way stations provided at spaced points along said route, a station on said vehicle, means providing for space communication between said vehicle station and different ones of said way stations when said vehicle is disposed at different points along said route, means at said vehicle station for signaling said dispatcher station over different routes which respectively include the nearest ones of said Way stations as said vehicle moves along said predetermined route, and means at said dispatcher station for establishing communication channels between said dispatcher station and said vehicle station over routes which respectively include different ones of said way stations.

4. A system for communicating between a dispatcher station and a vehicle moving along a Dredetermined route, comprising way stations provided at spaced points along said route, a station on said vehicle, means providing for space com- 0 munication between said vehicle station and different ones of said way stations when said vehicle is disposed at different points along said route, means at said way stations selectively controllable from said dispatcher station for establishing one way communication channels between said dispatcher station and said vehicle station over routes which respectively include different ones of said way stations, and means at the way station included in any one of s..id channels and controllable from at least one of the vehicle and dispatcher stations for changing the direction in which signals may be transmitted over the channel.

5. A system for communicating between a dispatcher station ,and a vehicle moving along a predetermined route, comprising way stations provided at spaced points along said route, a station on said vehicle, means providing for space communication between said vehicle station and different ones of said way stations when said vehicle is disposed at different points along said route, means at said dispatcher station for selectively seizing said way stations to establish one way communication channels between said dispatcher munication "channel,

assaggi station and said vehicle station, and means at any y'seized one of said way stations controllable from said dispatcher station for changing the direction Iof signal'transmission over the established corn- 6. A. system for communicating between a vvdisl-pa'tcheristation and a'vehicle'moving along a the ranges of communication of dierent pairs of said way stations as saidy vehicle proceeds along said route, and means for selectively establishing communication channels b etween'said dispatcherA station and said vehicle station over routes which respectively include the way station nearest or next nearest said vehicle. f

7. A system for communicating'with a vehicle moving along a-predetermined route, comprising a master station, a plurality of way stations located at spaced points along said route, a signal channel commonly associating said master station with said way stations, said'way stations each including space communicating equipment for radiating signals to a vehicle moving along said route and for ,receiving signals radiated from the vehicle, and means controllable from said master station over said signa'l channel for selectively' associating said way .stations with said-channel for the transmission of signals between said master station and thespace communicating equipment 'at any selected way stations.

8. 'A system for communicating with a vehicleV moving along a predetermined route, comprising a master station, a plurality of way stations located at spaced points along said route, a signal 'channel commonly associating said master station with said way stations, said way stations each including space communicating equipment for radiating signals to a vehicle moving along said route and for receiving signals radiated from the vehicle, means controllable from said master station over said signal channel for selectively associating said way stations with saidv channel for thel transmission of signals in onevdirection between said-master station and the space communicating equipment of the selected way station, and means at the selected way station, also controllable over said signal channel, for controlling the equipment at the selected way station to change the direction in which signals maybe transmitted between the master station and the selected lway station.

9. A system for communicating with a vehicle movingvalong a predetermined route, comprising a master station, a plurality of way stations located at spaced points along said route, a signal channel commonly associating said master -stalaying signals radiated from said'vehicle to said channel, means at said master station for selectively transmitting control signals over said channel having frequencies respectively corresponding to the different way stations, and means at said.

signals of ditlerentfrequencies 'for selectively #associatingsaid way stations with said channel v "-fo'rthe transmission of signalsibetween said master station and the space communicating equipymentat any selected way station; s l

l0. 4A 'system for communicating with 'a vehicle moving along a predetermined route, comprising a master station,' a plurality of way vstations located at spaced points along said route, a two- 'conductor signal channel commonly associating saidmaster stati'onwit-h said way stations; said waystations each including space communicating equipment for radiating signals received over said channel to a vehicle moving along said route and for yrelaying signals radiated from-said vehicle lto said channel, means at said master station for selectively transmitting control signals over said channelA having frequenciesnre'spectively correspondingvto the diiferent way stations, means at said way stations selectively responsive to said control signals'of different' frequencies for selectively associating said way stations with said channel for the transmission of signals received by the equipment at the selected way station to said master station ove'r said channel, and means each'including space communicating equipment for radiating to a vehicle' moving along said 4route signals` having frequencies within a predeter- `mined frequency -bancland received over said signal-channel and for relaying ignals`radiated from said vehicle to said chann l, meansV at said master station for -selectively transmitting control signals over said channel having frequencies outside said ypredetermined frequency bandand respectively corresponding to the different way stations, and means at said way stations selectively responsive to said control signals of different frequencies for selectively associating said way stations with said channel for the transmission of signals between said master station and the space communicating equipment at any selected way station. A

12. A system for communicating with a vehicle moving, along a predetermined route, comprising a master station, a plurality of way stations located at spaced pointsalong said route, a twoconductor signal channel commonly connecting said way stations with'said master station, said waystations each including space communicating equipment for radiating to a vehicle moving along said route signals having frequencies within a predetermined frequency band and received over fsaid signal channel from said master staation, and. for relaying signals radiated from said '.vehicleto said channel, means at said master station for selectively transmitting control signals over said channel having frequencies outside said predetermined frequency band and respectively corresponding to `the .different wayV stations, means at said way stations selectively. responsive vto said control signals of diiferent frequencies yfor selectively associating Vsaid way stationswith way stations selectively responsive to said control saidv channel *forlthe .transmission of signals received by the equipment at the selected way station to said master station over said channel, and means at the selected way station controllable over the twoconductors of said channel in parallel for selectively conditioning the equipment at the selected way station for-the radiation of sig-- cated along said route. a station on said vehicle,

means at said vehicle and way stations providing a space communication link between said vehicle and way stations, a general purpose communication channel associatingv said master station with said way station, and means at said way station controllable over said space link from said vehicle station for permitting said communication channel to b e monitored from said vehicle station over said space link.

14. A system for communicating between a master station and a vehicle moving along a predetermined route, comprising a way station located along said route, a station on said vehicle, means at said vehicle and way stations providing a spacecommunication link between said vehicle and Way stations, a generalpurpose communication channel associating said master station with said -way station, and means at said way station controllable over said space link from `said vehicle station for establishing a, calling connection between said vehicle and master stations which includes said space link and over which a callsignal may be transmitted to said master station, and for then altering the connection to establish a monitoring connection between vehicle and way stations over which said communication channel may be monitored from said vehicle station.

l5. A system for communicating between a master station and a vehicle moving along a predetermined route, comprising a way station located along said route, a station on said vehicle, means at said vehicle and way stations providing a space communication link between said vehicle and Way stations, a general purpose communication channel associating said master stalink,- means at said way station responsive to said control signal for conditioning the equipment at` l,said way station to relay signals radiatedi'rom 'said vehicle station to said communication channel, thereby to permit call signals to be transmitted to said communication channel from said vehicle station, and means operative following the operation of said last-named means for conditioning, for a period of-limited duration, the

equipment at said way station to relay signals received over said communication channel to said space link, whereby said communication channel may be monitored from said vehicle station.

17. A system for communicating between a master station and a vehicle moving along a predetermined route, comprising a way station located along said route, a station on said vehicle, space communication equipment at said vehicle and way stations providing a space communica,.- tion link between said vehicle and way stations, a general'purpose communication channel associating said master station with said way station, means atV said vehicle station for transmitting a control signal to said waystation over said space link,means at said way station responsive to said control signal for conditioning the .equipment at said way station to relay signals radiated from said vehicle station to said communication channel, thereby to permit call signals to be transmitted to said communication channel Ifrom said vehicle station, means operative following the operation of said last-named means for conditioning, for a period of limited duration, the equipment at said way station to relay signals over said space link from said communication channel, whereby said communication channel may be monitored from said vehicle station, and means at said way station controllable over said communication channel from said master. station for establishing a lcommunication connection between said master station and said vehicle station which includes said communication channel and connection between said master station and said vehicle station which includes said communica` tion channel and space link in series.

16. A system for communicating between v a master station and a vehiclemoving along a predetermined route, comprising a way station located along said route, a station on said vehicle, space communication equipment at said vehicle and way stations providing a .space communication link between said vehicle and way stations, a general purpose communication channel associating said master station with said way station, means at said vehicle station for transmitting a control signal to said way station over said space space link in series.

18. A system for communicating with a vehicle moving along a predetermined route, comprising a dispatcher station, a plurality of way stations located at spaced points along said route, a sta- Ytion on said vehicle, a general purpose communication channel commonly associating said dispatcher station with said way stations, said way stations and `said vehicle station each including space communicating' equipment for radiating signals and receiving 'radiated signals, whereby different space communication links may be established between said vehicle station and diiferent ones of said way stations, and means controllable from said dispatcher stationover said communication channel for selectively establishing communication channels between said dispatcher and vehicle stationsover routes which respectively include different selected ones of said space links.

19. A system for communicating with a vehicle moving along a predetermined route, comprising a dispatcher station, a plurality of way stations located at spaced points along said route, astation on said vehicle, a ygeneral purpose communication channel commonly associating said dispatcher station with said way stations, said Way stations and said vehicle station each including space communicating equipment for radiating signals and receiving radiated signals, whereby diilerent space communication links may be established between said vehicle and way stations, means at each `way station controllable over the associated space linkfrom said vehicle station for permitting said communication channel to be tion link between said vehicle land way stations,

a general purpose communication channel associating said master station with said way station.

means at said vehicle station for transmitting a control signal to said way station lover said space link, means at said way station responsive to said control signal for conditioning the equipment at said way station to relay signals radiated from said Vehicle station to said communication channel, thereby to permit call signals to be transmitted to said communication channel from said vehicle station, and means responsive to the termination of said control signal for conditioning the equipment at said Way station to relay signals received over said communication channel to said vehicle station whereby said communication channel may be monitored from said vehicle stafr tion.

ROBERT A. CLARK, JR. 

